new build choice, 2nd gen 50w

[norburybrook]

Phil,

It's just a screw same as if it were a ground screw. Only difference is it has a 3 hole terminal strip on it.

I personally can't see how it could come loose any more than a dedicated ground.

M

Sent from my Nexus 6 using Tapatalk

[pompeiisneaks]

Phil,

It's just a screw same as if it were a ground screw. Only difference is it has a 3 hole terminal strip on it.

I personally can't see how it could come loose any more than a dedicated ground.

M

Sent from my Nexus 6 using Tapatalk

I don't disagree, just indicating that a direct screw with only a grounding lug is less 'movable' than the terminal strip itself. I would suspect he had occasion to see the terminal strip come loose so he added that ground to ensure it doesn't ever come loose. Technically, if you did it like I think you did, the body of the regulator should also be touching chassis and screwed down which acts as both a heat sink and a ground, so you've really got 3 ground references.

My theory still stands but has no solid proof ;P

~Phil

[norburybrook]

Ok, got my two 9v batteries today and started with the FET adjustments.

I've got 18v at the top of the 22k and 8v at the bottom of the 10k using a 2.2k resistor.

How close do I need to get to 9v? and what value do I need to use from the 2.2k/ I don't think I have anything close to that at hand. I've tried a 1.8k and it dropped to 6v so I presume the 2.2k needs to go higher.

edit; Added a 360R in series and I've got 9.2v I presume that's close enough So If I now power the board from the B+ I should be looking to change the 22k to make sure I have 20v at the 10k/22k divider node?

M

[martin manning]

It’s the 8k2 going to ground that you need to adjust to get 20v at the top of the 10k drain resistor. The 8k2 is the lower part of a voltage divider with the 150k. Interesting that 2k seems to be just about right for the source resistor in several recent builds.

[norburybrook]

I've got a 22k going to ground with the 150k as per your FET pdf.


last thing it says is:

Adjust 22k resistor to get ~20V at top of Rd when powered by B+5




I have the 150k 2 w with a 22k and 10k.



M

[martin manning]

I was referring to your picture a few pages back... you must have changed that? If so you’ll be close with the 22k. BTW, the divider is the 150k and 22k. The 10k is the drain resistor, pretty much the same function as the plate resistor in a triode stage.

[norburybrook]

you have a good memory Yes I did indeed change it.

I realise that the divider is the 150/22k now, it's because I have the 22k and 10k together on by board they 'look' like a divider..if you get my drift. the 22k and 150k are miles apart on the board so don't even look connected.

I'm just wiring up the runs to the front now wher the relay and pot will be. I've ben using the inner sheaths of a Vann Damme shielded multicore balanced cable which seems to be perfect as it's 1; very thin 2; studio grade quality and 3: malleable.

DSC_1660.JPG

M

[Mr. dB]

One thing I've noticed about this 50W chassis is that it's cut for a Twin power transformer (3 3/4" x 3"), not a Bassman (3 1/2" x 2 3/4"). One is left with the choice to either use a transformer that fits, or cut a reducer plate.

Is there any particular harm in using a Twin-type power transformer in a 50W amp? The voltages are mostly the same, although the amp would draw about half the current the transformer was designed to deliver. I've seen this done in a couple of Small Special builds on this board.

.

Looks like some Marshall 50W type power transformers will fit, like the Classictone 1202-118.

[norburybrook]

I've used these InMadOut transformers on all my builds and they're so great I put up with the of size. Having a nice Twin PT with a dedicated 5v secondary for the relays is great. Also they sound amazing and are silent. My first build was with Hammond transformers and I got used to hearing a 'hum' when the amp was switched on.


My first build with the InMadOut transformers, I thought they were broken...Completely silent...zero hum..nothing...and that was in a recording studio with a really low noise floor


M

[dancemyth]

Hey guys. I've loved reading about this build. Congratulations so far, Marcus. Looks great!
I've been wrestling with a similar FET switching problem, so this has been really helpful for me. My problem now is that I've been unsuccessfully trying to download Martin's explanation for battery testing an FET circuit to set the bias (http://ampgarage.com/forum/viewtopic.php?f=5&t=18389). I find a bazillion references to this post, so I assume it's current, but only get a gibberish php file instead of a pdf attachment. Am I doing something wrong, or is this file somewhere else? Thanks a bunch. And good luck finishing up.
Dan

[martin manning]

May be an error- Download the file and chang the extension to .pdf and see if that works.

[dancemyth]

Doh! Thanks.

[pompeiisneaks]

It's a random bug with chrome I believe, the site itself is giving the pdf file, but the browser decides to name it the index.php name instead. Yes just renaming as mentioned fixes it.

Seems to only happen with PDF's so far as I can tell.

~Phil

[norburybrook]

Hello all. I'm home for a couple of days, so back soldering.

So... I have my parts for the FET gain pot and the footswitch.

Do I wire the switch on the Pot the same as the OD relay? looking at #124 the OD and PAB have different wiring for the switches, the PAB doesn't seem to ground. I presume I'll wire the switch on the back of the 10k pot like the OD relay with the center pins being the feed from the relay power, either the Top or bottom a ground, and the other to the footswitch.

if I want the Pedal setting to be with the Pot in it's normal 'in 'position which end do I wire to ground? if that is indeed the way to wire it.

M

[martin manning]

Sounds like you have it. It’ll be like the OD rear panel switch on the 124 scheme. Take the FET relay coil wire to the center (common) of the pull switch, and the normally closed terminal (pot pushed in) to the foot switch jack so the foot switch has control. Take the normally open terminal to the relay supply ground (by way of the OD switch) so when you pull the pot switch out the relay coil gets grounded.